A known swivel frame for a swivel work machine such as a backhoe includes a swivel base plate supported on a traveling machine body to be pivotable about a swivel axis and mounting an engine at a rear portion thereof. At a front end of the swivel base plate, a support bracket projects forwardly to support an excavator. On the swivel base plate, there is provided a partition wall which partitions the space forwardly of the engine in the fore/aft direction. Further, on the swivel base plate, there are provided a pair of right and left vertical ribs that interconnect the support bracket and the partition wall (see e.g. Japanese Patent Application “Kokai” No. 2006-144389 and Japanese Patent Application “Kokai” No. 2006-143004). In the swivel frame of this conventional type of swivel work machine, the swivel base plate is constituted of a single plate member having a uniform thickness throughout.